This invention relates to electronically tuned filters, and more particularly to improvements therein.
In a U.S. Pat. No. 3,811,101 there is described an electronically tunable waveguide resonator in which spaced irises are inserted to establish two capacitances in series at each iris location, together with switch means in the form of PIN diodes for selectively shorting out one of the series connected capacitances, whereby tuning may be accomplished. Since the tunable resonator is loaded with shunt capacitances (between the two conductors of the transmission line) each of which is in series with a PIN diode, the RF peak power rating of the resonator is established by the RF voltage allowable across the back biased diodes in the most significant tuning iris. This allowable voltage depends on the thickness of the I layer in the junction and also on the nature of the passivation used on the surface of the semiconductor die. From the highest-rated diodes in production, a selection might be made permitting application of zero-to-peak voltages in the range 700 to 1000 volts.
In order to increase the peak power rating of the resonator, a series string of PIN diodes, such as three, can be used to triple the reverse voltage ratings, (bias, zero to peak RF, and reverse breakdown) and roughly one order of magnitude increase in RF power rating of the resonator should result. Of course each diode in the string requires a heat sink (two of them electrically insulating) along with low-inductance interconnecting straps and bleeder resistors must be included to assure equal division of the bias voltage applied to the diodes. This approach for increasing RF power rating is straight forward, but costly, because of the extra components and intricate assembly work required.
Using the above techniques, it might be possible to raise the RF power rating to the order of 1 K.W. However, the total reverse bias voltage for the diode string associated with the most significant tuning iris would be on the order of 3000 volts, and the required high-switching-speed, solid-state, control system would be very difficult to realize.